Phosphoric acid concentration



United States Patent PHOSPHORIC ACID CONCENTRATION Sydney Atkin,Springfield, N..l., and Sidney S. Prince, New York, N.Y., assignors toChemical Construction Corporation, New York, N.Y., a corporation ofDelaware Application December 19, 1955, Serial No. 553,840

4 Claims. (Cl. 23-165) This invention deals with the concentration ofaqueous phosphoric acid contaminated with compounds of silicon andfluorine, such as the weak acid obtained by digesting ground phosphaterock with sulfuric acid or mixtures of sulfuric and phosphoric acids.Phosphoric acid obtained from this and similar sources is in widecommercial use for the production of superphosphate fertilizer, forwhich purpose it must be concentrated to a P content of about 50% orhigher. It is a principal object of the invention to provide an improvedconcentrating process for such acid wherein losses due to volatilizationof phosphoric acid during the concentrating step are largely avoided. Afurther object is the provision of an efiicient method for recoveringthe siliconand fluorine-containing compounds volatilized from thephosphoric acid during its concentration as commercially pure potassiumor sodium silicofluoride.

Aqueous phosphoric acid is usually concentrated by contacting it with astream of hot gases in a drum concentrator. This procedure utilizesfully the heat content of the gases for evaporating water and alsoremoves a substantial part of the siliconand fluorine-containingimpurities from the acid, but some of the phosphoric acid is alsovaporized or entrained and lost from the concentrating process. Thepresent invention provides a method whereby this loss of phosphoric acidin the stack gases from the concentrator is avoided so that the overallefliciency of the concentrating process is materially improved.

In its broadest aspects the process of our invention consistsessentially in scrubbing the stack gases from a drum-type phosphoricacid concentrator with a stream of strong aqueous phosphoric acid ofrelatively high P 0 content, whereby the volatilized phosphoric acid isrecovered from the stack gases, and introducing a portion of thescrubbing liquid into the concentrator. By this procedure substantiallyall of the phosphoric acid that would otherwise be lost in the stackgases is recovered in the product acid from the concentrator. We havefound that two very important advantages are obtained when the stackgases are scrubbed with a relatively strong phosphoric acid; thesiliconand fluorine-containing impurities in the gases are dissolved insuch a strong acid to a much smaller extent than in Water or weakphosphoric acid, and the hydrofluosilicic acid that is present in thestrong acid can be more completely precipitated by the addition ofWater-soluble sodium and potassium compounds thereto.

The solubility characteristics of hydrofluosilicic acid in aqueousphosphoric acid solutions must be understood for an accuratecomprehension of our invention. It is known that hydrofluosilicic acidis quite soluble both in water and in relatively dilute phosphoric acidhaving a strength up to about 19 percent H PO We have found, however,that in stronger phosphoric acid the solubility of hydrofluosilicic aciddecreases rapidly as the phosphoric acid content is increased, so thatwhen an acid having 21 P 0 content of 20% is used the fluorine contentcannot be more than about 9%. With still stronger acid thehydrofluosilicic acid solubility is still further decreased, and aphosphoric acid of 35% P 0 content saturated with hydrofluosilicic acidcontains only about 6% of fluorine.

As a result of this discovery it became evident to us that a strongaqueous phosphoric acid would function as a selective scrubbing agentfor removing substantially all of the volatilized phosphoric acid fromthe stack gases of a phosphoric acid concentrator while dissolving onlya minor proportion of the siliconand fluorine-containing impuritiestherefrom. Introduction of the resulting scrubbing liquid into theconcentrator would return practically all of the entrained phosphoricacid to the process while increasing only slightly the content ofcompounds of silicon and fluorine in the product acid. We found,however, that these impurities could be almost completely removed fromthe scrubbing liquid prior to its introduction into the concentrator, ifdesired, by a simple treatment with potassium chloride, sodium chlorideor other Watersoluble alkali metal salt and subsequent filtration orcentrifuging.

The invention will be further described with reference to theaccompanying drawing, the single figure of which is a flow sheetillustrating a preferred embodiment of the process and showing theapparatus used therein. On this drawing reference numeral 1 indicatesgenerally a phosphoric acid concentrator of the drum type; 2 is a mistagglomerator and entrainment separator for partially separatingentrained phosphoric acid from the stack gases and returning it to theconcentrator; 3 is a scrubber for removing phosphoric acid mist from thegases; 4 is a cyclonic separator for disengaging the scrubbing liquidfrom the gas, and 5 is a water scrubber wherein residual siliconandfluorine-containing impurities may be removed from the stack gasesbefore they are discharged to the atmosphere.

In the concentrator 1 a body of aqueous phosphoric acid 10 isconcentrated by the passage of a stream of hot gases therethrough. Thesegases are injected into the acid through a refractory-lined pipe 11which may be connected to any suitable furnace. In the embodimentillustrated a burner 12 is mounted directly over the concentrator 1 andthe products of combustion are introduced directly into theconcentrating drum, but it will be understood that a separately mountedfurnace maybe used if desired. The hot gases from sucha furnace arefrequently diluted with air to obtain an inlet gas temperature in thepipe 11 of about l200l400 F., but it is an important advantage of theinvention that inlet gas temperatures of 1500 F. and higher, andpreferably within the range of l5002200 F. may be used. Such hightemperature gases are not ordinarily suitable for drum concentrators asthey cause excessively high losses of acid through mist formation, butwith the mist-recovery process of the present invention they may beemployed with a corresponding increase in the thermal efliciency of theconcentrating process.

The moisture-laden gases from the concentrator leave through the stack13 at a temperature which is ordinarily Within the range of about225-275 F., the exact temperature depending on the strength of theproduct acid. These gases are discharged tangentially into the lowerportion of the mist agglomerator 2 which is an unpacked cylindricaltower wherein the gases ascend through a spiral path while entrainedliquid phosphoric acid separates out and is returned to the concentratorthrough line 14. During the passage of the gases and vapors through thisvessel the average size of the mist particles is increased byagglomeration, which is an important factor in the removal of these mistparticles from the gases in the subsequent scrubbing and separatingsteps.

lFliomqthe aggle nerator lithe gases pass through pipe 15 into the inletof a scrubber 3. The scrubbing apparatus illustrated on the drawing is aventuri scrubber of the type shown and described in US. Patent No.2,604,-

"1.85 ;.inth-is apparatus the gases areaccelerated to a rela .tivelyhigh linear .velocityina contraction-section .16 and are-then passedinto athroat section 17.provided withjets .for theinjectionof-streams-of scrubbing liquid transverse to the direction offlow of the gases therein. It will be understood that scrubbingequipment of other types may .be used in practicing theinvention, but aventuri scrubber .is preferred as it operatesat high efficiency with aminimum quantitytof scrubbing liquid.

The mixture .of gases and scrubbing liquid from the .throat 17 of :theventuri3 is decelerated in an expansion section 18 and then passesthrough an outlet pipe 19 into itheseparatorA. :Inthis separator, whichis an unpacked cylindrical chamber having a tangential gas inlet, the.gases .and vapors again pass upwardly through a spiral path while thescrubbing liquid containing dissolved phosphoric acid and impuritiessettles out by gravity and is .drawn off :through ;line 20. The gasesleaving the separator through pipe 21 may be.discharged directlyto the.atmosphereor they may be .further scrubbed with water inthefinalscrubber 5 by means of Water sprays 21 before being discharged throughstack 22.

Ordinarily no water -is.condensed from the stack gases when they arescrubbed with aqueous phosphoric acid solution because of the relativelyhigh temperature of the entering gases. The phosphoric acidconcentration in the scrubbing liquid is therefore increased by the acidwhich it removes from the gas. When the desired high P content hasbeenattained the scrubbing liquid is preferably diluted with asufficient quantity of water to maintain a constant concentration ofphosphoric acid and the excess scrubbing fluid is withdrawn andintroduced into the concentrator. By this procedure the phosphoric acidobtained from the stack gases is recovered in the product acid.

Referring again to the drawing, the scrubbing liquid from the separator4 is passed through line 20 into a recirculating tank 25,.from.which itis elevatedto the scrubber 3 by pump 26 and line 27 and reintroducedinto the throat of the scrubber through line 28. This recyclingprocedure is continued until a phosphoric acid concentration of at least20% P 0 and preferably about 25- 30% P 0 has been attained after whichwater is introduced through line 29 and a corresponding quantity of thescrubbing liquid is withdrawn through valve 30 and line 31 into thestorage tank 32 which feeds the drum concentrator. The quantity of waterintroduced through line 29 is just suflicient to maintain a constant P 0content in the'recirculating scrubbing liquid, and by thisprocedure thequantity of phosphoric acid returned to the concentrating step,isexactly equal to that scrubbed from thegases.

The above-descn'bed procedure is the best embodiment of the process ofour invention for concentrating plants operating on weak phosphoric acidobtained by digesting ground phosphate rock with sulfuric acid, sincethis acid usually contains considerable quantitiesof calcium sulfate insolution or as finely divided crystals. When a pure aqueous phosphoricacid from other sources containing at least 20% P 0 is beingconcentrated the gas scrubbing process can be modified, if desired, byinjecting a stream of said acid from the storage tank 32 into the throat17 of the venturi scrubber instead of the recirculating stream of acidentering through the line 28. In this case the separated scrubbingliquid in line 20 would be returned to the storage tank 32, or would besent directly to the concentrator 1 or, .if desired, it could beintroduced into the recirculating tank 25 and returned one or more timesthrough the venturi scrubber .whilea side stream is introduced into theconcentrating system through line 31.

These and other modifications to adapt the process to --the requirementsof varying types of feed acid will readily be suggested to those skilledin the art by the foregoing description, and are included within thescope of the invention.

When it is desired to purify the vw'thdrawn scrubbing liquid beforepassing it'to the concentrator, the valve 30 is closed and a side streamis withdrawn from the tank 25 throughline 33 and pumped by pump 34intothe tank 35. In this tank the acid is mixed with a quantity ofwater-soluble potassium or sodium salt suflicient to carry out thereaction This reaction goes to-substantial completion by reason of thevery low solubility of potassium or sodium silicofluoride in phosphoricacid having a P 0 content of 20% or higher, and therefore the treatedphosphoric acid is substantially completely free from dissolvedhydrofluosilicic acid. The stream of purified acid andprecipitateis-drawn ofi through line 36 and passed to a filter orcentrifuge 37, from which the potassium or sodium silicofiuoride isrecovered as a solid product of good purity. The'phosphoric acid, whichis removed as filtrate through line 38, has a very low fluorine contentand therefore may be returned to the concentrator 1 to be recovered inthe concentrating process which materially ,decreases the-fluorinecontent of the product acid.

From the foregoing description it will be seen thatthe process of theinvention provides animportant improvement in the concentration ofaqueous phosphoric acid. By scrubbing the stack gases from a drumconcentrator with aqueous ,phosphoric acid having a P 0 content of 20%to 35% or higher a substantially complete recovery of the volatilizedphosphoric acid in the gases is obtained as a solution containingonlyabout 69% of hydrofluosilicic acid. This solution can beintroduced intothe concentrator in quantities corresponding to the phosphoric acidremoved therefrom .in the stack gases with only a slight increase-in thefluorine content of the product acid and with a very substantialincrease in the overall efficiency of the concentrating process. Ifdesired, however, the hydrofluosilicic acid can be more easily andcompletely removed from this scrubbing liquid by treatment withwater-solublesodium or potassium salts than is possible with a moredilute scrubbing liquid, and therefore the purificationof the scrubbingliquid and the recovery of alkali metal silicofluorides therefrom isalso improved.

The advantages obtained by the present invention are furtherdemonstrated ,by thefollowing specific examples.

EXAMPLE 1 The process of the invention was tested in a plant in whichdilute phosphoric acid is concentrated to a P 0 content of 50-55% andthen reacted with ground phosphate rock for the production ofsuperphosphate fertilizer. The concentrating drum was designed toevaporate '100 tons of water per 24 hour day from a feed acid of about22-25% P 0 content using hot gases from a separate furnace which enteredthe drum at about 2000 F. and at a rate such as to produce a How ofapproximately 10,000 cubic feet perminute of gas at about 255 F. in theinlet pipe 15 of the scrubber. The linear gas speed in the venturithroat 17 was from 275 to BOO-feet per second and from 40 to 60 gallonsper minute of scrubbing liquid were injected into this throat throughline 28; the temperature of the gas and scrubbing liquid in pipe 1? wasabout F.

Using .a feed acid to the concentrator having a P 0 content of 23.5% anda fluorine content of 1.7% the concentration of hydrofluosilicic acid inthe scrubbing liquid was determined asits P 0 content increased bycontinued recirculation and the specific gravity and fluorine contentwere found to be as follows:

Acid in Scrubbing Liquid Specific Percent Gravity Fluorine PercentPercent Table I Feed Scrubber HaPOl Weir H3PO4 Drain Product Box (Line20) Percent Percent Percent Percent 22. 6 22. 8 57. 8 23. 3 1. 75 7. 531.19 4. 2 21. 8 22. 8 50. 3 22. 8

The plant was also operated for several days with water as the scrubbingliquid in the line 28 and with no admixture of scrubbing liquid with thefeed acid. With this procedure the concentrated acid analyzed 53.0% P 0and 1.12% F on one typical day of operation and 51.0% P 0 and 1.04% F onanother day, but the overall loss in phosphoric acid from theconcentrating process increased from practically nothing to about 5-6%.It is evident, therefore, that the process of the invention increasesmaterially the efficiency of the phosphoric acid concentrating processwhile increasing only very slightly the fluorine content of the productacid.

The following analytical figures show that the quality of thesuperphosphate fertilizer is not adversely affected when acidconcentrated by the new process is used for reaction with groundphosphate rock.

Triple superphosphate Made From When the process is operated forrecovery of the hydrofiuosilicic acid in the recirculating scrubbingliquid the valve 30 is closed and the pump 34 is operated to withdraw astream of the solution into the tank 35 at a rate equal to the additionof water through line 29. The solution in tank 35 is treated by addingthe amount of potassium chloride, sodium chloride or other watersolublepotassium or sodium salt theoretically necessary to react with thehydrofluosilicic acid in solution according to the equation Theresulting slurry is passed through line 36 to a centrifuge or filter 37where the purified acid is separated from the precipitate and returnedto the concentrating drum by admixture with the feed acid entering thisdrum.

When a scrubbing liquid containing 25% of P 0 and a quantity ofhydrofluosilicic acid corresponding to 7.0% fluorine was treated withpotassium chloride and filtered the filtrate was found to contain only0.05% of fluorine. It is evident, therefore, that when the concentratedphosphoric acid is to be used for purposes other than the manufacture ofsuperphosphate fertilizer the hydrofiuosilicic acid can be completelyremoved from the withdrawn stream of scrubbing liquid before it isreturned to the concentrator. By this procedure a substantial increasein concentrating efliciency is obtained without increasing the fluorinecontent of the product acid or, if desired, the purified scrubbingliquid can be separately concentrated.

What we claim is:

1. A method of concentrating a dilute impure aqueous phosphoric acidcontaining compounds of silicon and fluorine as impurities to a productphosphoric acid while simultaneously eliminating said impurities whichcomprises establishing in a drum-type concentrator a body of dilutephosphoric acid having a 2035% P 0 content, passing a stream of hot gasthrough the dilute impure phosphoric acid in a concentrating stepwhereby hot gas introduced into the body of said dilute phosphoric acidproduces a product phosphoric acid having a P 0 content of at least 50%and a stack gas containing said impurities along with volatilizedphosphoric acid and water vapor, said stack gas leaving the phosphoricacid being concentrated at a temperature of between 225 F. and 275 F.,thereafter scrubbing said stack gas with an aqueous phosphoric acidhaving a P 0 content of at least 20% and not over 35% thereby removingsaid volatilized phosphoric acid from said stack gas while dissolvinglimited quantities only of siliconand fluorine-containing compoundstherein, and recycling a portion of the resulting scrubbing liquid tothe concentrating step.

2. A method according to claim 1 in which the hot gas being delivered tothe concentrating step is at a temperature within the range of about1500-2200 F.

3. A method according to claim 1 in which the gas stream from theconcentrating step containing water vapor and impurities and at atemperature of between 225275 F. is accelerated and there issimultaneously injected into the accelerated gas a scrubbing liquidwhich is a recirculating stream of aqueous phosphoric acid having a P 0content of at least 20% and not over 35%.

4. A method according to claim 1 in which a portion of the scrubbingliquid, having dissolved therein limited quantities only of siliconandfluorine-containing compounds, is purified by adding thereto a watersoluble compound of a metal of the group consisting of potassium andsodium, removing the resulting precipitate, and passing purified acid tothe concentrating step.

References Cited in the file of this patent UNITED STATES PATENTS1,133,840 Collett Mar. 30, 1915 1,264,510 Hechenbleikner Apr. 30, 19182,611,681 Bellinger Sept. 23, 1952 2,655,431, Allen et a1. Oct. 31, 1953

1. A METHOD OF CONCENTRATING A DILUTE IMPURE AQUEOUS PHOSPHORIC ACIDCONTAINING COMPOUNDS OF SILICON AND FLUORINE AS IMPURITIES TO A PRODUCTPHOSPHORIC ACID WHILE SIMULTANEOUSLY ELIMINATING SAID IMPURITIES WHICHCOMPRISES ESTABLISHING IN A DRUM-TYPE CONCENTRATOR A BODY OF DILUTEPHOSPHORIC ACID HAVING A 20-35% P2O5 CONTENT, PASSING A STREAM OF HOTGAS THROUGH THE DILUTE IMPURE PHOSPHORIC ACID IN A CONCENTRATING STEPWHEREBY HOT GAS INTRODUCED INTO THE BODY OF SAID DILUTE PHOSPHORIC ACIDPRODUCES A PRODUCT PHOSPHORIC ACID HAVING A P2O5 CONTENT OF AT LEAST 5/%AND A STACK GAS CONTAINING SAID IMPURITIES ALONG WITH VOLATILIZEDPHOSPHORIC ACID AND WATER VAPOR, SAID STACK GAS LEAVING THE PHOSPHORICACID BEING CONCENTRATED AT A TEMPERATURE OF BETWEEN 225* F. AND 275* F.,THEREAFTER SCRUBBING SAID STACK GAS WITH AN AQUEOUS PHOSPHORIC ACIDHAVING A P2O5 CONTENT OF AT LEAST 20% AND NOT OVER 35% THEREBY REMOVINGSAID VOLATILIZED PHOSPHORIC ACID FROM SAID STACK GAS WHILE DISSOLVINGLIMITED QUANTITIES ONLY OF SILICON- AND FLUORINE-CONTAINING COMPOUNDSTHEREIN, AND RECYCLING A PORTION OF THE RESULTING SCRUBBING LIQUID TOTHE CONCENTRATING STEP.